1. Field of the Invention
The invention is directed to a method and apparatus for sensing conditions on an object. In particular, the invention is directed to a method and apparatus for determining at least one of pressure and temperature conditions on an object.
2. Description of the Art
It is desirable and useful to measure various characteristics, such as temperature and pressure on objects under operating conditions, to provide data for optimizing design details. The objects include operating rotating machinery comprising elements such as turbine parts including fans, compressor blades, turbine blades, vanes, etc. in jet engines,
It is relatively difficult to measure either pressure and temperature conditions on complex and intricately shaped objects, such as in rotating turbine parts under operating conditions. Most sensors have a bulk and disturb the aerodynamic flow that, of course, causes the pressure, and adversely impacts temperature on the object. Measuring stationary conditions on an object does not provide acceptable data to optimize designs when the machinery is operating.
It has been suggested that a sensor, such as a pressure sensor, be attached to or near the rotating parts. However, a pressure sensor has a definite volume and bulk, and the positioning of such a sensor will disturb flow during operating conditions and not reflect exact pressure data on the object. If the pressure and flow are disturbed, and an exact pressure data is not obtainable, a resultant design of the rotating parts will not take into account the true pressure on the rotating parts.
One method, which has been proposed to measure pressure conditions on complex-shaped parts without disturbing aerodynamic flow, uses a "pressure-sensitive paint". The pressure-sensitive paint is essentially a luminescent material, whose luminescent properties are affected by pressure. A known organic material that is used for the pressure-sensitive paint is pyrene, which is an organic compound whose luminescent lifetime is sensitive to the presence of oxygen in its local environment.
The pressure-sensitive paint is coated onto an object and optically interrogated with a light beam, typically by directing a short-pulsed laser beam at the pyrene containing organic material. A time-dependence of luminescence from the pyrene containing organic material is monitored, for example using a photodetector. As atmospheric pressure on the object increases, partial pressure of any present oxygen increases. Thus, more oxygen diffuses into the local environment of the pyrene containing organic material.
An increased concentration of oxygen in the pyrene containing organic material leads to a decrease in the luminescent lifetime of the pyrene. As pressure increases, the luminescent lifetime of the pyrene decreases. This relationship allows a determination of pressure by monitoring the luminescent lifetime of the pyrene containing organic coating material.
However, the use of the pyrene containing organic coating material is limited to low temperature environments, since degradation of the organic "paint" containing pyrene occurs at temperatures above about 300.degree. C. Additionally, a response time for this method is limited, both by the luminescent lifetime and a speed at which oxygen diffuses into the coating. Therefore, the use of the pyrene containing organic coating material is not effective for determining pressures conditions on rotating parts for turbines, where temperatures are routinely above about 300.degree. C., and further where a relatively rapid response time is needed because of changes in pressure over time.
Another attempt to determine pressure conditions on an object attempts to use a micromachined sensor array, which measures by optical analysis surface pressures. However, the array discussed in A Micromachnined Sensor Array for Optical Measurements of Surface Pressure, Miller et al., American Institute of Aeronautics and Astronautics (Jan. 1997) is separately constructed and then attached to the object. Thus, there will be irregularities in the surface due to the numerous arrays that are attached, for example by gluing, to the object. These irregularities do not reflect the true pressure on the object. Further, the arrays are sealed by its upper surface in a drum-like fashion, and thus in reality, depend on deflection of its upper surface to determine pressure acting on the array. The performance of this array is limited by the physical characteristics of the upper surface.
Accordingly, it is relatively difficult to accurately measure pressure on complex and intricately shaped objects, such in rotating turbine parts, including blades and disks. Presently, there are no acceptable and satisfactory methods for measuring pressure data under operating conditions, which does not disturb the aerodynamic flow.